When Is Less More? Examining Low-Power, High Function MEMS Accelerometer Options

Contributed by Mike Stanley

Originally posted on Freescale’s Smart Mobile Devices Embedded Beat Blog

Back in June and July, we discussed the MMA9550L Xtrinsic Motion Sensing Platform, which breaks new ground in the area of intelligent sensors (see Evolving Intelligence with Sensors and The Zen of Sensor Design).

Because the MMA9550L CPU is fully programmable, it’s a snap to add new features in software. Design cycles can be shortened and you can easily add features that distinguish your product from those of your competitors. But what if you don’t need the flexibility? Or your #1 competitive feature is price? Or power? In these cases, you may not want to pay any premium (no matter how small) for the on-chip MCU and memory that make the MMA9550L so flexible. That’s where the newest members of the Xtrinsic family, the 14-bit MMA8451Q and 12-bit MMA8452Q, come in. These devices, announced this week, utilize the same low-G MEMS transducer as the MMA9550L. But now the MEMS device is coupled with a cost reduced state-machine-based digital controller.

But don’t equate “cost-reduced” with “function-reduced”. For many applications, the MMA845xQ will be the sensor family of choice. Like the MMA9550L, they incorporate a number of intelligent features. The difference is that in the MMA845xQ devices, those features are implemented in hardware rather than software. Specifically, the MMA845xQ support:

  • the same ±2g/±4g/±8g dynamic range as the MMA9550L
  • 2 interrupt output pins / 7 interrupt sources
  • directional shake
  • tap detection
  • jolt detection
  • free fall detection
  • Portrait/Landscape detection
  • auto-wake-up and return-to-sleep

Freescale Xtrinsic Sensor MMA845xQ Block Diagram

The MMA845xQ devices are available in a small 3x3x1 mm QFN package and include an on-chip regulator, supporting supply voltages from 1.95V to 3.6V for the core supply voltage. They also have a separate I/O supply pin which supports 1.62V to 3.6V digital interfaces.

Noise and power numbers for these devices are phenomenal. Because they use dedicated logic circuitry, with no need for general purpose CPU and memory blocks, state-machine-based sensors normally use less power and generate less noise than their MCU-based brethren. Current consumption ranges from just 6 microamps at low data rates (12.5Hz and below) to 165 microamps at full data rates (up to 800Hz). Standby current is typically < 2 microamps. Even more impressive is the 99 micro-g’s/root Hz typical output noise you’ll see when the device is operating in its lowest noise mode.

The MMA845xQ family was designed to optimize not just the sensor, but also the overall system power consumption, for best in class performance for mobile phones, remote controls and gaming. They can be configured to automatically increase or decrease sample rate (along with current consumption) based on interrupt functions of the device. Even neater is that they can also send interrupts to the host controller indicating that it might want to consider changing its power mode as well. So if your phone is sitting motionless on your desk, it will know that fact, and be able to take advantage of it to dial down power. And when you pick your phone back up, it will know that as well, and kick start any tasks that need to be active when you are.

The 14-bit MMA8451Q has a 32 sample circular FIFO with programmable watermark that can optimize communications with your host processor. You can configure the FIFO to store any number of those 32 samples for data leading up to a trigger event (tap, Portrait/Landscape, transient or motion), with the remaining entries used to store data occurring immediately after the trigger event. When your event occurs, you get an interrupt and a window of data about that event in time. Very smart.

The features above were implemented in a digital controller significantly smaller than the MMA9550L, allowing the MMA845xQ devices to be price competitive for any application. MMA845xQ devices are sampling now. In 100K-piece quantities, prices are expected to be $0.95 for the MMA8451Q and $0.85 for the MMA8452Q.

Addition of the MMA8451Q and MMA8452Q to its portfolio demonstrate not only Freescale’s continued commitment to the MEMS marketplace, but also its recognition that no one sensor will be optimal for all applications. Use the MMA9550L when your low-G application requires an intelligent sensor hub or you need to incorporate your own custom sensor algorithms. Use the MMA845xQ devices when you can live with the extensive MMA845xQ feature set, and price and/or power are more important to your application.

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